[1.8.x] Fixed #24220 - Allowed lazy objects for success_url

Backport of 511be35779 from master

django/views/generic/edit.py

@@ -165,6 +165,8 @@ class ModelFormMixin(FormMixin, SingleObjectMixin):
         Returns the supplied URL.
         """
         if self.success_url:
+            # force_text can be removed with deprecation warning
+            self.success_url = force_text(self.success_url)
             if PERCENT_PLACEHOLDER_REGEX.search(self.success_url):
                 warnings.warn(
                     "%()s placeholder style in success_url is deprecated. "
@@ -299,6 +301,8 @@ class DeletionMixin(object):
 
     def get_success_url(self):
         if self.success_url:
+            # force_text can be removed with deprecation warning
+            self.success_url = force_text(self.success_url)
             if PERCENT_PLACEHOLDER_REGEX.search(self.success_url):
                 warnings.warn(
                     "%()s placeholder style in success_url is deprecated. "

tests/generic_views/views.py

@@ -172,9 +172,7 @@ class SpecializedAuthorDelete(generic.DeleteView):
     queryset = Author.objects.all()
     template_name = 'generic_views/confirm_delete.html'
     context_object_name = 'thingy'
-
-    def get_success_url(self):
-        return reverse('authors_list')
+    success_url = reverse_lazy('authors_list')
 
 
 class BookConfig(object):